Modern-day locomotive controls, including the locomotive brake control system, incorporate computer technology to reduce hardware and to facilitate adaption of the system to various customer requirements. In one such brake control system, a cab mounted, handle operated, brake command controller outputs the desired brake command signal to a microprocessor unit, which interprets the brake command signal in terms of a feedback signal indicative of the air pressure in an equalizer reservoir, and then effects operation of application or release electro-magnetic valves to adjust the equalizer reservoir pressure in accordance with the brake command signal.
A high capacity pneumatic relay valve device is employed to vary the trainline brake pipe pressure in accordance with variations of the equalizing reservoir pressure, in order to control the railway car brakes. A so-called brake pipe control circuit of the afore-mentioned brake control system is shown and described in the above-cited references, which are incorporated herein by reference.
The brake control system further includes an automatic brake control circuit having electro-pneumatic application and release valves, as well as an independent brake control circuit which includes additional electro-pneumatic application and release valves. The automatic brake control system is primarily intended for use when a train consist is made-up, for applying the brakes to all cars as well as the locomotives therein, while the independent brake control system is primarily intended for use when one or more locomotives are driven independent of any cars connected therewith when the automatic brake pipe lines may not be interconnected. The automatic brake control electro-pneumatic valves are operated by the microprocessor in response to changes in brake pipe pressure initiated by the brake pipe control circuit in accordance with movement of an automatic brake handle of the brake command controller. Another high capacity pneumatic relay device regulates the pressure in the locomotive brake cylinders according to the pressure output (brake pipe pressure) of the automatic brake control circuit application and release electro-pneumatic valves.
The electro-pneumatic valves in the brake pipe control circuit and in the locomotive brake cylinder control cylinder are arranged to assume a pressure release state, in the event of a power loss at the microprocessor unit. In consequence of such a power loss, therefore, brake pipe pressure is reduce while, concurrently, the locomotive brake cylinder pressure is released. A pneumatic backup control valve in the locomotive automatic brake control circuit is provided to establish locomotive brake cylinder pressure in response to the aforementioned reduction of brake pipe pressure resulting from such fail-safe operation of the electro-pneumatic valves in the brake pipe control circuit, there being a double check valve to separate the pneumatic backup control valve from the electro-pneumatic valves in the locomotive brake cylinder control circuit.
The pneumatic backup control valve includes a piston valve assembly subject on opposite sides to compressed air in the brake pipe and in the control reservoir. When brake pipe pressure is reduced, the resultant pressure differential forces the piston valve assembly to an application position, wherein the control reservoir air supplies the brake cylinder pilot line to establish the locomotive brake pressure until a force balance is restored across the piston valve assembly. In this manner, the piston valve assembly seeks a lap position in which the supply of brake cylinder pressure is terminated at a value corresponding to the brake pipe pressure reduction in effect.
The locomotive brake cylinder pressure may be released independently of the car brakes by means of a quick release valve associated with the pneumatic backup control valve. A pressure signal supplied to the quick release valve, when a quick release switch is activated, initiates this "bail-off" or quick release function. The brake pipe and control reservoir pressures are communicated via the quick release valve to establish pressure equalization across the piston valve assembly, when the quick release switch is actuated. In this manner, the control reservoir pressure is effectively equalized with the reduced brake pipe pressure, such that a spring force acting on the piston valve assembly, is effective to force the piston valve assembly to release position and the locomotive brake cylinder pressure is exhausted.
In order to reapply the brakes, the quick release switch is deactuated to interrupt communication between the brake pipe and control reservoir pressures via the quick release valve, so that a subsequent reduction of brake pipe pressure is effective to produce a further pressure differential across the piston valve assembly. In response to this further pressure differential, the piston valve assembly will return to the application position and reestablish the supply of control reservoir pressure to the brake cylinder pilot line to reapply the locomotive brake.
In the event of a power failure at the microprocessor, brake pipe pressure is reduced to zero due to the fail-safe configuration of the electro-pneumatic valves in the brake pipe control circuit, which would, therefore, allow the locomotive to be moved without the ability to obtain any automatic locomotive braking whatsoever. The above-noted U.S. Pat. No. 5,222,788 discloses a circuit modification to correct this shortcoming by providing a microprocessor means having a regulated source of electrical power for controlling the first electro-pneumatic valve means in accordance with the brake command signal, and a brake assurance means for establishing an energized condition of the second electro-pneumatic valve means in response to a loss of the regulated source of electrical power.
In addition to the above, it had been found that in the event of a power failure, it was also possible to inadvertently trap or discharge the air pressure in the independent application and release pipe, controlled by the independent brake circuit, which thereby causes the air in the independent application and release pipe to be inadvertently trapped if the brake was applied at the time of power failure, or to be inadvertently discharged if the brake was not applied at the time of power failure. If air were trapped in the independent application and release pipe, air would be trapped at an equal pressure within the locomotive brake cylinders, causing the locomotive to remain with it brake applied, which would not allow it to be moved. On the other hand, if the air were discharged from the independent application and release pipe, the independent brake control system would not function if the locomotive were moved. While the above referenced patent teaches a system that will permit the automatic brake system to be functional in the event of a power failure, there are times, in switch yards for example, when the brake pipe may not be interconnected, so that the automatic brake control system cannot be used. In those situations, therefore, a power failure can disactivate the only functional brake control system; i.e., the independent brake control system.